The number of UK-born children identified as being trafficked for sexual abuse more than doubled last year.

Data released by the National Crime Agency (NCA) showed that 56 minors who are from the UK were flagged up as potential victims of trafficking for sexual exploitation in 2013, a rise of 155% on the previous year.

It is not clear whether they were being taken out of the country or moved within the UK, the NCA said. The number of foreign children identified as being at risk also rose by 11%, to 88.

For children the most common nationality or country of origin was Vietnam, followed by the UK and then Albania.

The figures are the number of cases highlighted under the National Referral Mechanism (NRM), used to bring possible victims to the authorities' attention.

The NCA said that 1,746 people from 112 different countries were highlighted as as potential victims of traffickers last year , up 47% on the previous year.

Nearly two thirds of those referred were female (1,122) and around a quarter (450) were children.

In total the number of cases involving Britons or the UK as country of origin rose 173% to 90; of those, 63 were children.

Liam Vernon, Head of the NCA's UK Human Trafficking Centre, said: "Increased awareness, both of human trafficking in its various forms and the obligation of first responders to use the National Referral Mechanism, is a likely contributor to the increased number of referrals in 2013.

"We know that this is a crime which affects some of society's most vulnerable people, and some victims will remain undetected. Equally, some of those referred to the NRM may not ultimately be classified as victims of human trafficking.

"The NCA is committed to relentlessly disrupting what is a criminal trade in human misery."

Of the number of minors identified as potential victims, 45 were classed as possible domestic servitude (up 2% on 2012), 123 as labour exploitation (up 24%) and 138 were unknown (up 9%).

There was one case where age was unknown, as was the exploitation type.

Albania was the most common nationality or country of origin for all referrals, followed by Nigeria and Vietnam.

Home Office minister Karen Bradley said: "Today's figures are unlikely to show the full scale of modern slavery nor the human suffering behind each statistic.

"That is why we are taking action on a number of fronts including raising public awareness. The National Crime Agency is leading an enhanced and co-ordinated response to targeting trafficking gangs, we are increasing protection for victims, and we are strengthening legislation through a Modern Slavery Bill.

"The Bill will send the strongest possible message to criminals that if you are involved in this disgusting trade in human beings, you will be arrested, you will be prosecuted and you will be locked up."

Deputy executive director of Unicef UK Anita Tiessen said: "Unicef UK is alarmed to see an increase in this worrying trend. Trafficked children - regardless of whether they're born in the UK or born elsewhere and trafficked into the UK - face violence, exploitation and abuse of the most unimaginable kind.

"The upcoming Modern Slavery Bill offers the UK a chance to develop world- leading anti-trafficking legislation, and Theresa May must seize this opportunity to protect trafficking's most vulnerable victims - children."